1. Field of the Invention
The present invention relates to color correction in digital images and, more particularly, to color correction in printed digital images.
2. Related Art
Various kinds of printers are well-known in the computing and digital image arts. Such printers include, for example dot-matrix printers, laser printers, inkjet printers and thermal printers. Thermal printers, for example, use thermal energy (heat) to produce printed output. More specifically, thermal printers typically contain a linear array of heating elements (also referred to herein as “print head elements”) that print on an output medium by, for example, transferring dye/pigment from a donor sheet to the output medium or by initiating a color-forming reaction in the output medium. The output medium is typically a porous receiver receptive to the transferred dye/pigment, or a paper coated with the color-forming chemistry. Each of the print head elements, when activated, forms color on the medium passing underneath the print head element, creating a spot having a particular density. Regions with larger or denser spots are perceived as darker than regions with smaller or less dense spots. Digital images are rendered as two-dimensional arrays of very small and closely-spaced spots.
Color thermal printers may include either a single thermal print head or multiple thermal print heads, each of which is responsible for printing a distinct colorant. In a four-head printer, for example, the four print heads may be responsible for printing cyan, magenta, yellow, and black, respectively. The print heads typically are spaced some distance apart from each other in a row or other configuration.
The medium on which output is to be printed (referred to as the “output medium,” “web,” or “receiver”) typically is provided on a continuous roll, referred to as the “receiver roll.” The receiver is pulled from the receiver roll through the printer by a drive capstan roller located after the final print head. In this manner the receiver passes by and makes contact with each print head in succession. Each print head transfers dye/pigment of a corresponding colorant from a donor element to the receiver as the receiver passes by it. In this way, a four color image may be printed by successively printing each of four single-color layers on the output medium. The processes of printing distinct colors of an image at successive print stations is referred to as “tandem printing.”
Consider a color digital image to be printed by a color printer. For purposes of example assume that each pixel in the image has 8-bit red, green, and blue (RGB) component values. In an ideal color printer, the relationship (referred to as the printer's “tonescale”) between RGB values in the digital image to be printed and the resulting colors in the printed image would be both user-selectable and fixed over time.
The tonescales of real printers, however, do not remain fixed over time, but rather vary due to factors such as temperature and variations in the output media. Furthermore, even the tonescale of a single model of print engine may vary from engine to engine due to variations in the manufacturing process. As a result, color output may vary in accuracy over time within a single printer or from printer to printer in undesirable ways.
The process of modifying the tonescale of a color printer to compensate for such variations and, more generally, to improve the perceived quality of colors in the printed output is referred to as “color calibration” or “color correction.” In general, color correction typically is initiated by printing a test image. The color characteristics of the test image are measured using a measurement device such as a densitometer, calorimeter, or spectrophotometer, and the measured characteristics are compared to desired characteristics to determine what, if any, adjustments are needed to be made to the printer's tonescale to re-optimize printer performance. These adjustments are then applied to the printer by altering settings such as the printer's lookup tables (LUTs).
High-quality color correction is becoming increasingly important as color printers become used more frequently for printing photo-quality images and as the expectations and sophistication of users increase. High-quality color correction is also particularly important in printers, such as printers used as commercial photo kiosks, that are expected to produce a high volume of high-quality output and for which manual adjustments are inconvenient or unduly burdensome. What is needed, therefore, are improved techniques for performing color correction in a color printer.